The present invention relates to chemical recovery furnaces and particularly to apparatus for controlling dampers adjacent ports where combustion air is introduced into the firebox of the furnace.
Wood pulp for paper making is usually manufactured according to the sulfate process wherein wood chips are treated with a cooking liquor including sodium sulfide and sodium hydroxide. The wood chips and the cooking liquor, called "white liquor", are cooked in a digester under predetermined heat and temperature conditions. After cooking, the used liquor, termed "black liquor", containing spent cooking chemicals and soluble residue from the cook, is washed out of the pulp and treated in a recovery unit where the cooking chemicals are reclaimed. Without reclamation and reuse of the cooking chemicals, the cost of the paper making process would be prohibitive.
In the recovery process, the black liquor is first concentrated by evaporation to a water solution containing about 65% solids, which solution is then sprayed into the firebox of a black liquor recovery boiler, a type of chemical reduction furnace. The chemical reduction furnace is a reactor wherein the processes of evaporation, gasification, pyrolysis, oxidation and reduction all occur interdependently during recovery of the cooking chemicals. The organic materials in the black liquor, lignin and other wood extracts, maintain combustion in the firebox, and the heat produced dries and melts the spent cooking chemicals as they fall to the floor of the firebox, where they build a mount of material called a char bed. The char bed is further heated to further liquefy the chemicals into a molten smelt that flows out of the furnace through a smelt spout to a collection tank. Concurrently, combustion heat is employed to generate steam in boiler tubes for use as process steam and for generating electricity.
The combustion process requires the introduction of large volumes of air into the firebox, air comprising about 80% of the material entering the firebox. The air is forced into the firebox from windboxes or ducts disposed at several levels in surrounding relation to the firebox, through a plurality of air ports in the walls of the furnace, viz.: primary, secondary and tertiary air ports. The primary air ports, through which about 40 to 50% of the air enters the furnace, are disposed on the sidewalls of the firebox near the bottom of the furnace close to the char bed. The secondary air ports, through which about 35% of the air enters the furnace, are disposed around the walls of the firebox, higher than the primary air ports, and closer to the entry conduits through which the black liquor is sprayed into the firebox. While the primary air ports provide a large volume of air with considerable turbulence for maintaining a fireball in the char bed, the secondary and tertiary air ports provide fine control and distribution of air above the char bed and distribute the air evenly in the black liquor spray.
The black liquor sprayed into the firebox, having a consistency similar to that of warm sixty weight oil, swirls, burns and falls toward the bottom of the firebox as combustion products comprising char material and smelt. The smelt and char material contact the outer walls of the firebox and, cooled by the inflowing air, form excrescent deposits around the edges of the air ports, particularly along the edges of the openings where the excrescent material builds up, and around the openings under influence of air rushing through the air port. Such build-up of char material can block the desired air flow and can even block individual ports completely. In accordance with customary practice, the char build-up is periodically removed by a worker inserting a rod into the air ports successively around the boiler. With manual rodding of the air ports, gradual build-up of char material intermittently around the furnace causes changes in the volume of combustion air, as well as changes in air distribution, velocity and pressure. Moreover, the manual rodding may not always remove as much of the char material as would be desired. Particularly hard deposits of solidified smelt and char material resembling a vitreous substance resist cleaning. Therefore, furnace operation often tends to be inefficient and unpredictable with an attendant decrease in the amount of chemical that can be recovered, a decrease of steam produced per unit of fuel, and increased emission of noxious gasses such as carbon monoxide and sulfur dioxide.
Much more satisfactory cleaning operation can be accomplished through the use of automatic powered apparatus positioned adjacent each air port for periodically ramming through the char build-up, for example, see Goodspeed U.S. Pat. No. 4,822,428 granted Apr. 18, 1989. Such apparatus employs a metal cleaning tip on the end of a rod wherein the rod is powered by a remotely controlled air cylinder. The mechanism indexes along the air port for the purpose of successively cleaning different areas thereof.
However, the primary air ports in some boiler installations are intentionally partially blocked by manually operated dampers located in the windbox adjacent the air ports and used for adjustably controlling air flow. These manually operated dampers are difficult to move from one position to another because of the appreciable air flow they control, and because of being constructed of fairly heavy material. Furthermore, the dampers adjacent primary air ports tend to remain in one position for long periods of time since fine air regulation is not required at the primary level. The dampers often become virtually immovable due to build-up of excrescent material.
The presence of dampers hinders hand rodding of primary air ports. Hand rodding may only be undertaken for the cross section of the air port which can be seen, i.e., that portion not covered by a damper, with resulting build-up of excrescent material adjacent the damper. Installation of power cleaning equipment such as described above is virtually prohibited by the presence of hand operated dampers since the dampers would need to be moved manually each time the power equipment is operated in order to avoid mechanical interference.
Totally automated cleaner-damper combinations have been employed, particularly at the secondary air port level. (See U.S. Pat. No. 4,838,182 and U.S. Pat. No. 4,846,080.) However, use of this type of equipment is not warranted at the primary level inasmuch as fine damper control is not required. Replacement of dampers in an existing boiler with costlier equipment represents an expense which may be unjustified at least from the standpoint of air control. A boiler user would obviously prefer to avoid major alteration of an existing boiler through replacement of equipment that normally performs its function. Indeed such replacement may not be possible without major shutdown and boiler remanufacture. Moreover, the dirtier environment at the primary level is not conducive to replacement of existing equipment with more complex apparatus as may be difficult to maintain.